1. Field of the Invention
The present invention relates to a plasma monitoring apparatus for monitoring the plasma condition of a plasma load supplied from a high frequency power source.
2. Description of the Related Art
Plasma is used for etching, sputtering, thin-film growth, and other plasma processes during the manufacture of semiconductor elements and liquid crystal displays. Plasma processes such as these supply a high frequency current to two electrodes to generate plasma between the electrodes inside the chamber where the specific etching, sputtering, thin-film growth, or other process occurs.
When a high frequency current is supplied to a plasma load in order to generate plasma in this manner, it is important to match the impedance of the high frequency power supply and the plasma load. When the impedances cannot be matched, power reflection at the high frequency supply output terminal inhibits the efficient supply of power to the plasma load. In such cases, good results cannot usually be expected from the plasma process.
It is therefore essential to insert an impedance matching circuit comprising an inductance circuit, capacitance circuit, and transformer between the high frequency supply and the plasma load when supplying power from a high frequency source to a plasma load.
FIG. 11 is a circuit diagram of a typical circuit for supplying power from a conventional high frequency supply to a plasma load. As will be known from the figure, power is supplied from the high frequency supply ("power supply" below) to the plasma load 5 via a coaxial cable 2, impedance matching circuit 3', and a power line 4. A typical impedance matching circuit 3' comprises, for example, a first variable capacitor C11, a coil 11 for adjusting the impedance by selecting a tap, and a second variable capacitor C22. Impedance matching is accomplished by adjusting the capacitance of the first variable capacitor C11 and second variable capacitor C22.
To automatically adjust the first and second variable capacitors C11 and C22, a detector detects the phase difference between the voltage and current on the power supply-side terminal of the impedance matching circuit 3', detects the impedance Zin from the power supply-side terminal of the impedance matching circuit 3' to the plasma load 5 side based on the ratio between the absolute value of the detected voltage and the absolute value of the detected current, and adjusts the capacitors C11 and C22 so that this detected impedance matches the output impedance Zout of the power supply, and the phase difference is zero.
The adjustment matching the impedance Zin to the output impedance Zout of the power supply is accomplished primarily by adjusting the capacitance of the first variable capacitor C11. The adjustment whereby zero phase difference is achieved is accomplished primarily by adjusting the capacitance of the second variable capacitor C22.
The plasma load 5 consists primarily of a chamber 5a, two electrodes 5b1 and 5b2, power supply units 5c1 and 5c2 for supplying current to the impedance matching circuit 3' and the chamber 5a, and power lines 4 connecting the is electrodes 5b1 and 5b2 with the power supply units 5c1 and 5c2. It should be noted that the power line 4 is shown in the chamber 5a, and will be known to also connect the load-side terminal of the impedance matching circuit 3' and the power supply units 5c1 and 5c2.
In plasma processes as described above, it is necessary to constantly monitor the plasma condition in order to achieve good reproducibility and improve the yield of the semiconductor elements being produced. It is also known that the plasma condition can be monitored by measuring the plasma impedance, for example, or the peak-to-peak plasma voltage.
There are two impedances present between the electrodes when plasma is generated when viewed from the load-side terminal of the impedance matching circuit 3' to the plasma load 5 side, that is, the internal resistance of the plasma, and the electrostatic capacitance of the sheaths generated between the plasma and the two electrodes 5b1 and 5b2. There is also impedance in the power line 4 connecting the load-side terminal of the impedance matching circuit 3' and the plasma load 5. It should be noted that it is often not possible to directly connect the load-side terminal of the impedance matching circuit 3' and the power supply units 5c1 and 5c2. A power line 4 of a specific length is used in this case.
An equivalence circuit for the section from the load-side terminal of the impedance matching circuit 3' to the plasma load 5 side when these impedances are present is shown in FIG. 13. Voltage vectors for this equivalence circuit are shown in FIG. 14. Note that VD is the terminal voltage on the load-side terminal of the impedance matching circuit 3'; VL is the power line 4 voltage; VR and VC are the voltage and combined electrostatic capacitance of the sheaths occurring at the internal resistance of the plasma; and VP is the voltage between the electrodes 5b1 and 5b2.
Because it is necessary to detect the voltage VP between the electrodes 5b1 and 5b2, it is usually necessary to pull a lead outside the chamber 5a for detecting this voltage VP. This usually requires modifying the chamber, which is a complicated task. As a result, it is usually the load-side terminal voltage VD of the impedance matching circuit 3' that is detected. The problem with this configuration is that voltage VD is affected by the length of the power line 4, that is, the voltage VL occurring in the power line, and the detected peak-to-peak voltage is therefore imprecise.
For example, a 13.56 MHz power supply is the high frequency supply most commonly used in plasma processes. With such a power supply, the electrostatic capacitance per sheath is 283 pF assuming a dielectric constant in a vacuum of 8.854.times.10.sup.-12, sheath permittivity of 1, an electrode area of 0.032 m.sup.2, and sheath thickness of 1 mm. The combined electrostatic capacitance of the sheaths is therefore approximately 142 pF, and the plasma reactance is -j82.7 .OMEGA. for a 13.56 MHz supply.
The relative precision of measuring the peak-to-peak voltage VDP for voltage VD, and measuring the peak-to-peak voltage VPP for voltage VP, can be calculated as 100.multidot.(VDP-VPP)/VDP (%). Measurements were taken and the precision calculated as described for the following parameters: plasma power supply, 1 kW; 10 .OMEGA. internal plasma resistance; and power line inductance of 100 nH, 500 nH, and 1 .mu.H.
At 100 nH, the precision was -11%; at 500 nH, -102%; and at 1 .mu.H, -707%. It will thus be obvious that precision drops as inductance increases.
There is therefore a need for a plasma monitoring apparatus whereby the precision of measuring the inductance or peak-to-peak voltage of the plasma load can be improved when monitoring the plasma condition of the plasma load.